Aspergillus fumigatus is the major invasive mold pathogen of immunosuppressed patients, causing exceptionally high morbidity and mortality. Weakened defenses as a consequence of hematopoietic stem cell transplantation or solid organ transplantation allow growth of inhaled spores in the lung and dissemination. Both therapy and diagnosis are problematic, a situation that contributes to extremely high mortality rates. Our objective is to use A. fumigatus gene expression during infection in a murine model to define virulence genes. We will prioritize genes for functional analysis based on in vivo expression and host response, and then validate the utility of the data through construction of new mutant strains and determination of their virulence potential. These gene products in turn will be high priority targets for therapeutic and diagnostic development. We will use a nanoString nCounter to quantify specific A. fumigatus RNA levels in infected tissue. NanoString estimates of RNA levels are much more sensitive than microarray estimates or current RNA-Seq capability. Our proposed studies focus on three classes of gene products: transcription factors, surface and secreted proteins, and secondary metabolite biosynthetic enzymes. We have chosen transcription factor genes because their products can be connected to target genes and biological functions through expression profiling and chromatin immunoprecipitation. We have chosen surface and secreted protein genes because their products present the most accessible therapeutic targets, and because their possible release into fluids makes them candidate diagnostic targets. We have chosen secondary metabolite genes because their products may have biological activity, such as immunomodulation, that is relevant to pathogenesis. We will develop a reference dataset that explores expression of these genes, and extend the analysis with functional validation through two specific aims. First, we will define A. fumigatus gene expression during lung infection, using a murine model and two sequenced A. fumigatus strains. Second, we will validate functional inferences from expression data through virulence assays of select mutant strains. Our findings will significantly improve the understanding of A. fumigatus infection by providing a set of regulatory pathways, surface/secreted products, and toxins that impact infection. The information will be critical for prioritizing pathways and gene products as targets for therapeutic and diagnostic development, and to connect basic research studies to gene products that are highly relevant to infection.